According to news reports, chain collision accidents on highways are met frequently. When a car is moving along the highway, it is not easy for the driver to estimate the distance kept between his car and the front car. If the distance between two cars is too short, then when the front car brakes abruptly the car behind may not be able to brake, causing accidents. But if the distance between two cars is too great, then the car behind will keep on pressing the horn or flashing the headlights to urge the front car to move faster. Although the traffic regulations stipulate that at a speed of 60 km/hr, a car has to maintain a distance of 6 car lengths from the front car, and at a speed of 90 km/hr, a distance of 9 car lengths. But it is difficult for the driver to judge how many car lengths there are between his own car and the front car. Moreover, because of health conditions, psychological factors, or a lack of concentration, drivers often fail to brake the cars and cause chain collision accidents. Nowadays there are so many cars in cities that when it rains there are certainly traffic jams. The cars move slowly and have to frequently and alternately stop and move, and the drivers have to keep on stepping on the accelerator, changing shifts, or applying the brakes; all this is not only time-consuming but also exhausting. Air pollution may also result (because the speed of acceleration is not easily controlled, combustion of gasoline may not be complete). What is more, collision may easily happen if the drivers are careless. Although the cars today are usually equipped with automatic shifting systems, the drivers still have to constantly step on the accelerator and/or brakes, while concentrating their attention on maintaining a safety driving distance. To the busy and nervous modern people, this is really very exhausting.